Keep your acrylic colours wet for weeks, months..
I tend to use alot of mixed colours,that i have made for spesific colour scheme, or even murals.
For murals, i find the best way to store large amounts of premixed paints is in old well washed shampoo bottles, with a Well sealing lid- or screw top sealing sauce bottles the squeezy kind. They seal well, and make pouring on a pallet very easy and mess free, they also come in clear which makes it easy to identify your colour!
For smaller amounts of paint, i tend to premix and entire colour scheme for a painting- i do this on a pallet or and old dinner plate. I scoop my colours into large ice trays with a pallet knife. This holds the paint in a little resivor which stops it drying out so quickly during use. Not only this when i pack it up i tend to squirt the tray with a fine mist of water from a house hold squirty bottle, before covering and sealing with glad wrap. Using this method i have kept small amounts of paint for months during summer. I occassionally unwrapped it and reapplied the spray but all was well.
Also for easy clean up of good qaulity acrylics, allow to dry completely on a non porus pallet, You will find It peels off in small sheets. It may take a little scratching to lift the edges. Test out your paints on an old dinner plate you dont care about if you are unsure, but you should be pleasantly surprised.
Hope that helps you out.